Extensive efforts are being made to maximize the production of light olefin such as ethylene and propylene in order to meet the growing demand of light olefins. As used in this specification the term light olefin is deemed to mean ethylene, propylene but not butylene. Generally, propylene and ethylene are produced as by-products (4-6 wt. % propylene and 2-3 wt. % ethylene) while manufacturing fuels such as gasoline, diesel and the like by employing the Fluid catalytic cracking (FCC) process. Propylene is separated from FCC reactor product vapors for getting petrochemical feedstock. However, the separation of ethylene from other FCC products is not very economically attractive due to the lower quantity of the same; therefore, it is used as a refinery fuel gas. Since late nineties, some FCC units have been operating at higher severity to get propylene of 10-12 wt % of the fresh feed. To further increase the propylene yield, different processes have been developed around FCC configuration. Some of them are still operational.
Existing Knowledge
U.S. Pat. No. 4,980,053 discloses a process called Deep Catalytic Cracking (DCC) wherein a preheated hydrocarbon feedstock is cracked over heated catalyst in a reactor to produce light olefins. The obtained gaseous products are separated into ethylene, propylene, butylenes and other components.
As, this process requires 5-10 seconds of contact time, it produces relatively substantial quantity of undesirable products like dry gas.
U.S. Pat. No. 5,846,402 discloses a catalytic conversion of heavy hydrocarbon feedstock to produce high yield of liquefied petroleum gas and light olefins having 3 to 4 carbons. The cracking of feedstock is carried out in a high velocity riser reactor under catalytic cracking conditions that include high catalyst to oil ratio and high riser temperature. The cracking catalyst as disclosed in aforementioned US patent is a solid acid catalyst comprising ultra stable-Y (USY) zeolite, shape selective pentasil zeolite and active matrix. The U.S. Pat. No. 5,846,402 teaches the art of cracking heavy hydrocarbon in a single riser configuration at very high severity.
Further, U.S. Pat. No. 7,491,315 discloses a process of cracking the hydrocarbon in a dual riser configuration. The heavy hydrocarbon like vacuum gas oil cracks in a first riser at relative lower severity to produce products; the products obtained from the first riser i.e. light crack naphtha and C4 stream are then allowed to crack in a second riser at very high reaction severity. The cracking catalyst is a combination catalyst having a conventional FCC catalyst in combination with ZSM-5 catalyst. The process as disclosed in U.S. Pat. No. 7,491,315 is a heat balanced process in which the heat generated by burning the coke deposited on catalyst is utilized further. However, the additional coke precursors are introduced in the riser to obtain heat-balanced condition. The major drawback of two-riser configuration is that it requires an additional catalyst circulation loop including new riser reactor which adds to increased capital expenditure and space.
Mehlberg et. al. in their United States Patent Application 2010/0168488 disclose a process of cracking multiple feedstock in a dual riser reactor vessel. The reason for opting two reactor vessels is argued to be an artifact of equilibrium composition governing the C3-C5 range olefins. While two separate reactors, two separate main columns and gas connection are good to achieve close to equilibrium yield of light olefin and avoid recycling of lower crack-able C4-C6 hydrocarbons, such scheme add to duplication of many equipment/vessel thereby leading to high capital expenditure. There are some “On-purpose” propylene production processes which convert low value naphtha feedstock to light olefins by employing ZSM-5 zeolite based catalyst at very high reaction severity. The light feed stocks make very less catalytic coke on ZSM-5 zeolite based catalyst; therefore, these processes require external heat supply to satisfy heat demand for the endothermic cracking in the riser. In general, fuel oil in combination with heavy oil product, produced in the process, is injected in the regenerator as a source of heat. But, the continuous burning of fuel in regenerator accelerates hydrothermal deactivation of catalyst. Further, some part of feed material is being continuously burned for supplying additional heat which leads to some bearing on process economics. U.S. Pat. No. 7,601,663 discloses a cracking of straight run naphtha to light olefins in a riser at very high riser temperature using ZSM-5 zeolite based catalyst. The heat balance is satisfied by preheating the feedstock in a separate furnace and by burning the fuel oil in a regenerator which further leads to higher hydrothermal catalyst deactivation resulting requirement of higher catalyst make up rate.
U.S. Pat. No. 5,171,921 discloses a new process for converting C3 to C20 hydrocarbons to light olefins using ZSM-5 zeolite catalyst having phosphorous. In this process, the production of catalyst coke is very less, therefore, the heat required for endothermal cracking is supplied partially by preheating the feedstock and continuous burning the fuel oil in a regenerator.
Further, U.S. Pat. No. 7,867,378 discloses sequential cracking of ethanol and hydrocarbon wherein ethanol converts to ethylene in a first reaction zone and hydrocarbons to light olefins in a reaction zone in a single riser. Usually, the riser bottom temperature and pressure are very high, ethanol over cracks to lighter gas. In case severity at riser bottom is less, conversion of hydrocarbon to lighters will also be less.
Further, U.S. Pat. No. 5,981,819 discloses a process (Propylur process) that employs adiabatic fixed bed type, similar to that employed in a claus unit, to convert olefin streams of C4 hydrocarbon to light olefin using ZSM-5 type of catalyst.
Other than aforementioned described prior-art processes, methanol conversion processes are also employed to produce light olefins e.g. propylene and ethylene. These conversion processes are characterized by a high exothermicity, depending especially on the methanol conversion rate. These processes employ mostly fixed bed reactor configuration and reactor design is determined by heat control and removal of heat from the process. As it is fix bed process, it needs regeneration in regular frequency.
U.S. Pat. No. 4,627,911 discloses a heat neutral cracking process where exothermic methanol cracking and endothermic gas oil cracking are combined to achieve heat neutrality. The process as disclosed in aforementioned patent is executed without regenerating the catalyst. However, this process requires methanol to be injected preferably upfront of the gas oil to increase the catalyst temperature before gas oil cracking. Also the residence time required in the riser is much higher >6-15 sec with corresponding low WHSV of ˜25 hr−1. However, to maximize light olefin, it is necessary to minimize riser residence time to minimize H-transfer reactions. Further, the higher temperature (>550° C.) overcracks the methanol leading to unwanted dry gas formation.
Bernhard Lucke et al. (Microporous and Mesoporous Materials 29 (1999) 145-157) disclose a method for simultaneous conversion of C4/N-hexane/naphtha and methanol over ZSM-5 zeolite based catalyst in a fix bed configuration. The combination of endothermic cracking of hydrocarbons and exothermic cracking of methanol having suitable weight ratio produces light olefins under thermally sustained conditions. However, the co-feeding of hydrocarbons and methanol results in 100% conversion of methanol, but less conversion of hydrocarbon. Bernhard further discloses that higher reaction severity increases hydrocarbon conversion, but over cracks olefin to methane. Perhaps, higher temperature and higher residence time over cracks methanol to lighter like methane. However, the co-feeding of hydrocarbons and methanol does not give optimum conversion of both methanol and hydrocarbon and optimum selectivity towards olefins.
Therefore, a process of catalytic conversion of hydrocarbon feedstock having wide range of hydrocarbon feeds in the single riser to produce high yield of lower olefins is provided in the present disclosure wherein the drawbacks of related prior-art processes such as use of a dual reactor system, ill production of desired light olefins and deactivation of catalyst are obviated successfully.